(Supported by NIH NCRR/ P41-RR 01219) The goal of this project is to determine the advantages of high-voltage EM relative to intermediate-voltage EM for reconstructing the high resolution ultrastructure of thick biological specimens. Various parameters including, e.g., specimen type, specimen preparation method, and EM imaging conditions, as well as subsequent data presentation (i.e., post-processing and reconstruction methods) are being explored. Both the variable-voltage capability of our HVEM, as well as our IVEM are being used in this project. The HVEM was set up to use Kodak SO-163 film, the standard film used in the other electron microscopes in our facility. This is necessary for these tests because images often must be recorded at low dose, and the Dupont "lo-dose" film we had been using for this is not suitable for optical diffraction, one of the tools to be used in assessing image quality. For both HVEM and IVEM, the standard push-processing method for SO-163 is being used for low-dose work, and normal development for routine work. Preliminary comparison studies of the same 1(m thick specimen imaged at 1000kV and 400kV in the HVEM showed detectable, but insignificant, differences in image quality only at 60( tilt. However, the difference in exposure between zero and sixty degrees was 3 times at 1000kV and 25 times at 400kV. The longer exposure time at 400kV made recording drift-free images problematical.